Olympic athletes arriving at Heathrow from next week face potential transport delays after the Highways Agency said it was unable to guarantee that urgent repairs on the M4 motorway would be completed by Monday.

The first section of the Olympics Road Network, in which segregated lanes are reserved for athletes and officials, was due to open on the motorway on Monday, which Olympic bosses expect to be one of the busiest days for athlete arrivals.

The Highways Agency said repairs to a damaged viaduct, due to be completed by this morning, had reached a critical stage and that "due to the complexity of the operation, more time than originally envisaged is required to complete the work safely".

A section of the main motorway link between Heathrow and London has been closed in both directions since last Friday after a crack was discovered in the Boston Manor viaduct.

A spokeswoman said the agency expected the work to be completed "in the next few days" but could not guarantee it would be completed by Monday. "This is the most sensitive part of the repair process and we have to get it right," she said.

The ongoing closure is an blow for Olympic organisers and Transport for London (TfL), who are anxious to avoid transport chaos across the capital during the Games period. The controversial Games Lanes stretching across 170 miles of roads, including 109 miles in the capital, are reserved for the exclusive use of athletes, Olympic officials, media and emergency vehicles. The remainder of the network is due to open on 25 July, two days before the opening ceremony.

Traffic, normally totalling 100,000 vehicles a day, is being diverted to the A4 and A312. A TfL spokeswoman said no provisions were in place to allow priority for Olympic vehicles on those routes, but added: "We're considering whether that might be something that we need to bring in."

She said TfL was also encouraging Olympic arrivals to use public transport. "At this stage they are not needing to get to their competition venues, they are just getting to their accommodation."

On Wednesday the Emirates Air Line, the new cable car across the Thames linking several Olympics venues, was closed for 33 minutes because of the risk of a lightning strike. "Due to the nature of this mode of transport there will be times when adverse weather conditions mean it will not be possible to operate," said TfL.

The hairline crack in the 1960s-built viaduct was discovered as part of ongoing maintenance inspections, the Highways Agency said. Sixty-three other small cracks had been identified in inspections since mid-May, most of which were repaired without closing the road, but the most recent fault was in a particularly sensitive location. The agency said 14 separate groups of workers were working across 24 hours a day to fix the fault.